Filters for placement in the vena cava are well known. A preferred filter shape is conical and sized such that the wide end of the cone is sufficiently large to be able to abut and press against the vessel wall, often fixed thereto by barbs or similar anchoring devices. The narrow end of the filter is typically closed. The filter is often formed as a mesh of wires or struts having apertures which are sufficiently large to allow fluid therethrough but small enough to trap debris and thrombus material. A conical shape has the advantage that the cone tends to expand as a result of blood flow pressing into the filter, which thereby assists in holding the filter in place.
A problem with such filter designs is that there is a risk of deploying the filter at an angle in the vessel, that is with the open end of the cone not properly aligned in the plane transverse to the vessel. Even if it is deployed accurately, there is the risk that the filter tilts over time. Any such tilt of the filter can result in loss of patency, or sealing, to the vessel wall and as a result loss of filtration function. Tilting can also cause loss of coupling to the vessel wall, which can result in migration of the filter over time. The art has sought to address these problems by modification of the delivery assembly, for instance to provide devices which seek to position the narrow end of the filter at the centre point of the vessel. Alternatively or additionally, the generally conical filter design may be modified by the addition of stabilising struts and the like, for instance by providing a double filter structure with opposing filter cones.
While a specially designed deployment assembly may reduce or substantially avoid the risk of incorrect placement of the filter in a vessel, it has no further function after deployment and as a result cannot assist in maintaining proper orientation of the filter after deployment.
The addition of further stabilising struts to a conical filter can add bulk to the structure, particularly when the filter is radially compressed for deployment purposes. Furthermore, known stabilising structures are such that they are liable to impinge on fluid flow, thereby risking the generation of thrombi. This can be particularly the case with double cone symmetrical filter designs.
Moreover, vena cava and other filters tend to be temporary medical devices requiring removal once the period of treatment has been completed. A problem with conical shaped filters lies in the lack of options with regard to retrieval, in that they normally must be retrieved from their narrow or closed end first. In addition, conical shaped filters tend to be harder to reposition once deployed in the patient's vessel.
The skilled person will appreciate that at least some of the above considerations can apply equally to occlusion devices of conical form.
Examples of filters can be found in WO-2006/074163, US-2012/0071914, WO-02/102280, US-2010/0030254, WO-95/13761 and German Utility Model DE-29/500,864U. Examples of embolic protection devices can be found in US-2010/0274277, US-2007/0066991, US-2007/0078481 and US-2010/0211094.